The present invention relates to a photocurable hygroscopic composition capable of forming a transparent and flexible hygroscopic layer, and an organic EL element comprising drying means formed of the composition.
An organic EL device utilizing electroluminescence (hereinafter referred to as “EL”) of an organic material comprises an organic layer provided by stacking an organic charge transport layer or an organic light-emitting layer between an anode and a cathode and is attracting attention as a light-emitting device enabling high-intensity light emission by low-voltage direct current driving. Also, all materials of the organic EL device can be formed of a solid matter and therefore, this device is promising as a flexible display.
On the other hand, the organic EL device has a problem that when the device is driven for a certain period of time, the light emission characteristics such as light emission intensity, light emission efficiency and light emission uniformity seriously deteriorate as compared with those at the initial stage. The cause of this deterioration in the light emission characteristics is, for example, oxidation of the electrode due to oxygen intruded into the organic EL device, oxidative decomposition of the organic material due to generation of heat at the driving, oxidation of the electrode due to free water in air intruded into the organic EL device, or denaturation of the organic material. Furthermore, for example, interfacial separation of the structure by the effect of oxygen or free water, generation of heat at the driving, or high temperature in the environment at the driving may trigger the generation of a stress at the interface of the structure due to difference in the thermal expansion coefficient between respective constituent elements, and the resulting mechanical deterioration of the structure, such as interfacial separation, may also give rise to the deterioration of light emission characteristics.
In order to prevent these problems, a large number of techniques of encapsulating the organic EL device and thereby inhibiting contact with free water or oxygen are being studied. For example, there has been disclosed a method where, as shown in FIG. 1, a sealing cap 2 having laminated on the inner wall thereof a water absorbent 6 is placed on a picture element area formed by arraying an organic EL device comprising a substrate 1 having thereon a transparent electrode 3, an organic functional layer 4 and a metal cathode electrode 5 and after filling the inside with a nitrogen gas, fixed to the substrate 1 with an adhesive 7, thereby preventing free water from reaching the organic EL device (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 9-148066). Also, a method of reducing the effect of oxygen by using an oxygen absorbent in place of the water absorbent has been disclosed (see, for example, Kokai No. 7-169567).
As for the water absorbent, various substances have been investigated and among these, an alkaline earth metal oxide such as barium oxide (BaO) and calcium oxide (CaO) is being widely studied because, unlike other water absorbents such as silica gel and zeolite of physically adsorbing water, a water molecule can be unfailingly captured by a chemical reaction and release of a water molecule at a high temperature does not occur.
However, such a water absorbent is an inorganic compound particle and requires a concave substrate for its attachment in a device, and this disadvantageously gives rise to increase in the thickness of the device. Furthermore, the alkaline earth metal oxide is opaque and may be applied to a so-called bottom emission-type display device of taking out the display light from the substrate 1 side, but in the case of application to a so-called top emission-type display device of taking out the display light from the sealing cap 2 side opposite the substrate 1, the emission of display light is inhibited by the water absorbent 4 and therefore, the water absorbent 4 must be disposed not to cover the picture element area, which imposes a limitation that the site for the placement must be newly provided.
For applying a water absorbent to such a top emission-type display device, several proposals have been made. For example, it may be easily thought to apply a transparent water-absorptive polymer such as polyvinyl alcohol and nylon as the water absorbent. However, these polymers physically adsorb water and their water absorptivity is not sufficiently high. Also, a technique of disposing a particulate water absorbent in an organic EL device of top-emission structure to an extent of not inhibiting the optical transparency (see, Kokai No. 2001-357973), and a technique of using a plastic substrate having dispersed therein a water absorbent having a particle diameter smaller than the light emission wavelength of the organic EL device (see, for example, Kokai No. 2002-56970), have been proposed. However, in either case, the water absorbent used is an inorganic particle and has a difficulty in the method for placement and also, reduction in the transmittance due to light scattering is inevitably caused because uniform dispersion to a primary particle is difficult.
In order to solve these problems, use of a water catching film having small visible light absorption has been disclosed (see, Kokai No. 2003-142256). The water catching film can be formed by solvent-coating a special metal compound and has sufficiently high transparency. However, this water catching film comprises a low molecular compound and therefore, disadvantageously lacks flexibility in the case of application to a flexible substrate.
On the other hand, the encapsulation process of an organic EL device is performed in an extremely dried glove box so as to prevent intrusion of free water and considering this point, a solventless type not using a solvent is preferred in view of safety and environment. However, the conventional water catching film is a solvent-coating type as described above, and a solventless type is demanded. As a solventless type water catching film, a film consisting of a mixture of a cyclic aluminum compound having radical polymerizable group and acrylic monomer has been disclosed (see, Kokai No. 2005-7235). However, this film has no flexibility and transparency.